The use of LED diodes in applications like displays, information and advertising panels, signs, traffic signals, automotive lighting is becoming more and more common and sophisticated.
A current driver device for LED diodes has to provide very high and accurate channel currents. The current accuracy of all the channels is of course the main common feature of all current driver devices, however, the modern LED array drivers are offering a large amount of extra features.
A typical LED array is constituted of a plurality of channels providing the output current required to turn on the LED diodes. A current driver for the LED array comprises a voltage reference, which usually is a buffered band-gap voltage. An external resistance is connected between the voltage reference and a ground reference GND, in order to generate a current used internally as an accurate current reference.
FIG. 1 shows a basic implementation of a current driver for LED diodes applied to a channel of the LED array. The reference current Iref flows through a resistance Rref generating a voltage drop Vref. An output current Iout flows through a series of LED diodes, connected between a voltage Vled and a first terminal of a switch M1 (for example a MOSFET device). A resistance Rsense is connected between a second terminal of the switch M1 and a ground reference GND, and a voltage Vsense is generated by the flow of current Iout through it. The switch M1 is driven by the output signal of an operational amplifier OPAMP configured to make the voltage Vsense (at the inverting terminal) equal to the voltage reference Vref (at the non-inverting terminal). The current Iref is mirrored to all the channels of the array.
The accuracy between different channels is affected by numerous parameters, for example the mismatch of current mirrors, the offset voltage of the operational amplifier OPAMP, the mismatch of resistances Rref and Rsense or the mismatch of the GND metal affecting the voltage Vsense. The chip to chip precisions, instead, are affected mainly by the band gap voltage reference Vref and the current mirror accuracy.
Currently the current driver for an array of LED diodes uses digital circuits in each channel for modulating the current flowing through the channel; this determines a large area of circuit consumption and reduced cost effectiveness.